1. Field of the Invention
The present invention relates to a lead-free glaze, a spark plug on which it is used, and a method of producing the spark plug.
2. Description of the Related Art
A spark plug insulator is an example of an insulator coated with a glaze. The glaze is coated onto the insulator after which the insulator is fired simultaneous to sealing parts in the hole in the insulator.
It is necessary that the conditions under which parts are sealed into the hole of this insulator are such that the temperature is no higher than 900xc2x0 C. to prevent oxidation of the stem.
There has been a demand for lead-free glaze in recent years to accommodate environmental concerns. However, it is difficult to fire lead-free glaze onto an insulator at a temperature of 900xc2x0 C. or lower. Thus, it is no longer possible to perform firing simultaneous to sealing parts in the hole of the insulator.
In consideration of this problem of the prior art, the object of the present invention is to provide a lead-free glaze that allows firing at low temperatures, a spark plug in which it is used, and a method of producing the glaze.
A first aspect of the present invention is a lead-free glaze for coating ceramic materials containing 16-49 wt % of SiO2, 15-35 wt % of B2O3, 0-10 wt % of Al2O3 and 0-10 wt % of ZnO.
As a result of having the above constitution, the lead-free glaze of the present invention can be fired onto an insulator even at a temperature of 900xc2x0 C. or lower. In addition, since this lead-free glaze does not contain lead, it is suitable for environmental protection.
In addition, since the lead-free glaze of the present invention can be fired at a low temperature of 900xc2x0 C. or lower, if the ceramic material is a spark plug insulator, oxidation of the stem inserted into the hole of the insulator can be prevented. Consequently, firing of the coated glaze and sealing of parts in the hole of the insulator can be carried out simultaneously.
The following provides an explanation of the composition of the lead-free glaze of the present invention.
SiO2 and B2O3 are the main components of borosilicate glass. Regarding SiO2 and B2O3, the melting point of a glaze tends to increase as the amount of SiO2 increases, and the ratio of SiO2/(SiO2+B2O3) is preferably 50-70 wt %. If the ratio is less than 50 wt %, the water resistance of the glaze decreases resulting in the risk of the glass component eluting in water and deteriorating. If the ratio exceeds 70 wt %, the melting point rises resulting in a risk of the smoothness of the glazed surface decreasing.
The content of SiO2 is 16-49 wt %. If the SiO2 content is less than 16 wt %, there is the risk of the water resistance of the glaze decreasing. If the SiO2 content exceeds 49 wt %, the melting point of the glaze rises resulting in a risk of the smoothness of the glazed surface decreasing.
The content of B2O3 is 15-35 wt %. If the B2O3 content is less than 15 wt %, the melting point of the glaze rises resulting in a risk of the smoothness of the glazed surface decreasing. If the B2O3 content exceeds 35 wt %, there is the risk of the water resistance of the glaze decreasing.
Al2O3 demonstrates the effect of improving the water resistance of the glaze when added in a minute amount, and prevents the glass component from eluting and deteriorating in water. The content of Al2O3 is 0-10 wt %. If the Al2O3 exceeds 10 wt %, the viscosity during firing increases resulting in the risk of the smoothness of the glazed surface decreasing. The content of Al2O3 is preferably 2-10 wt %. If less than 2 wt %, there is the risk of a reduction in the effect of improving the water resistance of the glass.
ZnO stabilizes the glass without increasing the viscosity during firing. In addition, ZnO also has the effect of suppressing increases in the coefficient of linear expansion of the glaze. The content of ZnO is 0-10 wt %. If the ZnO content exceeds 10 wt %, the transparency of the glazed surface becomes poor.
The above lead-free glaze also preferably contains one type or two or more types of components selected from the group consisting of CaO, BaO and MgO. This is because BaO, CaO and MgO stabilize the glass without increasing the viscosity during firing.
The above lead-free glaze also preferably contains one type or two or more types of components selected from the group consisting of Bi2O3, ZrO2, TiO2, CeO and FeO.
Although Bi2O3 lowers the melting point of the glaze, if added in large amounts, it results in the risk of the glazed surface losing it smoothness.
ZrO2 has the effect of stabilizing the glass and lowering the coefficient of linear expansion, while also increasing the ceramic strength. On the other hand, the addition of ZrO2 in large amounts causes clouding.
Although TiO2, CeO and FeO have the effect of preventing discoloration of the ceramic material by increasing weather resistance, addition of these components in large amounts conversely colors the glaze.
Thus, these components are preferably blended so that the necessary coefficient of linear expansion is obtained. As a result, the glass components in the glaze can be stabilized, discoloration of the ceramic material can be prevented, and the melting point of the glaze can be lowered.
Moreover, the above lead-free glaze also preferably contains one type or two or more types of components selected from the group consisting of Li2O, Na2O and K2O. Li2O, Na2O and K2O are alkaline metal oxides that lower the melting point of the glaze. The addition of these components improves the smoothness of the glazed surface.
The above lead-free glaze further preferably contains 2-30 wt % of BaO. BaO has a potent effect of suppressing increases in viscosity during firing, and results in a smooth glazed surface when added in an amount of 2 wt % or more. If the BaO content is less than 2 wt %, there is the risk of the viscosity of the glaze increasing. In addition, if the BaO content exceeds 30 wt %, there is the risk of an increase in the coefficient of linear expansion.
Moreover, the above lead-free glaze preferably contains 1-10 wt % of ZrO2. ZrO2 stabilizes the glass in the glaze and has the effect of lowering the coefficient of linear expansion. Consequently, the strength of the ceramic material can be increased by coating a ceramic material with glaze containing ZrO2. On the other hand, if the ZrO2 content is less than 1 wt %, there is the risk of an increase in the coefficient of linear expansion of the glaze, while if the ZrO2 content exceeds 10 wt %, there is the risk of the glaze becoming clouded.
The above lead-free glaze also preferably contains 1-25 wt % of Bi2O3. Bi2O3 has the effect of lowering the melting point of the glaze. If the Bi2O3 content is less than 1 wt %, there is the risk of reducing the effect of lowering the melting point of the glaze. If the Bi2O3 content exceeds 25 wt %, there is the risk of the glazed surface losing its smoothness.
The above lead-free glaze preferably contains the following components:
SiO2: 35-49 wt %
B2O3: 20-35 wt %
Al2O3: 2-10 wt %
ZnO: 0-10 wt %
BaO: 2-25 wt %
ZrO2: 1-10 wt %
Bi2O3: 1-15 wt % and,
at least one type of LiO2, Na2O or K2O: 0-10 wt %.
If the content of SiO2 is less than 35 wt %, the water resistance of the glaze decreases resulting in the risk of the glass component eluting in water and deteriorating. If the SiO2 content exceeds 49 wt %, the melting point of the glaze rises resulting in the risk of decreased smoothness of the glazed surface.
If the content of B2O3 is less than 20 wt %, the melting point of the glaze rises resulting in the risk of decreased smoothness of the glazed surface. If the B2O3 content exceeds 35 wt %, there is the risk of the water resistance of the glaze decreasing.
If the content of Al2O3 is less than 2 wt %, there is the risk of decreasing the effect of improving the water resistance of the glaze by addition of Al2O3. If the Al2O3 content exceeds 10 wt %, viscosity during firing increases resulting the risk of a decrease in the smoothness of the glazed surface.
If the content of ZnO exceeds 10 wt %, there is the risk of a decrease in transparency of the glazed surface.
If the content of BaO is less than 2 wt %, there is the risk of the effect of suppressing increases in viscosity during firing decreasing and, if the BaO content exceeds 25 wt %, there is the risk of an increase in the coefficient of linear expansion.
If the content of ZrO2 is less than 1 wt %, there is the risk of a decrease in the ceramic strength. If the ZrO2 content exceeds 10 wt %, there is the risk of clouding of the glaze.
If the content of Bi2O3 is less than 1 wt %, there is the risk of an increase in the melting point of the glaze. If the Bi2O3 content exceeds 15 wt %, there is the risk of the glazed surface losing its smoothness.
If the total content of at least one type of LiO2, Na2O or K2O exceeds 10 wt %, there is the risk of the coefficient of linear expansion of the glaze layer increasing, or insulation resistance at a high temperature of, for example, 500xc2x0 C., decreasing.
A second aspect of the present invention is a spark plug comprising coating the surface of the insulator with the above lead-free glaze and firing.
Since the spark plug has a lead-free glaze according to the present invention fired onto the surface of the insulator, firing can be performed at low temperatures enabling it to be manufactured safely. In addition, the glazed surface following firing is smooth. Consequently, the plug cap is attached easily. In addition, there is little adhesion of debris and insulation resistance can be secured from a normal temperature to 500xc2x0 C.
In addition, the lead-free glaze may be fired after adding minute amounts of clay component such as kaolin and bentonite or organic binder to the glass components. In this case, the composition of the lead-free glaze after firing is the same as the above-mentioned composition before firing.
If the insulator is made of alumina and if the glaze layer has a coefficient of linear expansion of 50-80xc3x9710xe2x88x927/xc2x0 C. that approximates that of the alumina, although firing can be carried out without cracking and so forth, the coefficient of linear expansion of the glass increases rapidly at the transition point. Thus, it is preferable that the coefficient of linear expansion of the glaze at 100-300xc2x0 C. be 50-75xc3x9710xe2x88x927/xc2x0 C. In the case of deviation from this range, there is the risk of the occurrence of cracks in the glaze.
A third aspect of the present invention is a method of producing a spark plug comprising coating the above lead-free glaze onto the surface of an insulator and firing at a temperature of 900xc2x0 C. or lower. In this production method, the insulator is coated with a lead-free glaze that allows the above low-temperature firing. Consequently, the lead-free glaze can be fired at a low temperature of 900xc2x0 C. or lower, allowing firing to be carried out at low cost.
A fourth aspect of the present invention is a method of producing a spark plug comprising coating the above lead-free glaze onto the surface of an insulator, inserting a part into the hole of the insulator and heating the insulator and part to simultaneously fire the above lead-free glaze and seal the above part.
The present production method uses a lead-free glaze that allows the above low-temperature firing. In the present production method, since firing of the lead-free glaze and sealing of the above part by sealing glass and so forth are carried out simultaneously, a spark plug can be obtained easily.
The heating temperature of the above insulator is preferably 900xc2x0 C. or lower. As a result, a spark plug can be obtained both easily and at low cost.